The invention relates to an apparatus for producing a solid aerosol, such as a carbon aerosol, in which the aerosol particles are produced by a spark discharge via particle-supplying electrodes and the particles are entrained by a gas flow.
Various means for producing a solid aerosol are known, e.g. particles can be removed from a column consisting of compacted carbon particles by a brush and introduced into a gas flow used for discharging the same. In order to produce a carbon aerosol a discharge across twp spaced carbon electrodes has been proposed. During the spark discharge the electrode material evaporates and individual particles condense in the gap between the electrodes. A gas flow flows around the electrodes and entrains the condensed particles and discharges them in aerosol form out of the apparatus. In the case of the latter apparatus it is only possible to produce aerosols with a limited concentration over a short time and on a laboratory scale since the electrodes "burn-up" and, consequently, the operating conditions change, so that no constant operation is possible.
The aim underlying the present invention essentially resides and further developing an apparatus of the aforementioned type so that, in adjustable manner over a considerable period of time, extremely constant particle concentrations can be obtained, with the apparatus being suitable for calibrating soot measuring devices, for filter tests and for inhalation experiments.
According to the invention, an apparatus for producing a solid aerosol, such as a carbon aerosol, is provided in which aerosol particles are produced by spark discharge via particle-supplying electrodes and the particles are entrained by a gas flow, with the electrodes having parallel adjacent end faces and a feed drive moving them synchronously opposite to one another. The inventive construction makes it possible to automatically keep the gap between the parallel facing front faces of the electrodes constant over a long period of time. Thus, a constant sparkover voltage and, consequently, a constant particle generator operation is obtained. The inventive apparatus in particular ensures that the electrodes always project symmetrically into the gas and particle supply channel and therefore the burn-up of the electrodes takes place centrally in the aerosol channel.
According to a preferred further development, the electrodes are driven by spindles with oppositely directed threads and, in particular, the electrodes are fixed to running supports, which are in engagement with spindles and the latter are driven by a motor via a toothed belt. As a result of this construction a uniform guidance of the electrodes is achieved, which is important for a uniform burn-up thereof and ensures a minimum change to the sparkover voltage. According to a further development it is possible to provide a regulating means for the electrode feed drive. Whereas fundamentally the electrode burn-up rate is constant, so that a controlled drive could be provided, this construction permits a broad adjustability of the particle concentration, in that in each case, the feed control is automatically locked as a function of the desired particle concentration and there is no need to provide different external control programs. In an extremely preferred development control takes place by a measuring device for the sparkover voltage between the facing sides of the electrodes. By measuring the sparkover voltage precisely the latter, whose constancy is sought, is used as a measured variable or given actual value for regulating the electrode gap and therefore in desired manner maintaining constant the sparkover voltage.
According to a further development of the invention a gas supply channel for carrier gas and particle flow is constructed in a PTFE body. As a result a minimum number of particles adheres to the aerosol channel, so that the discharged particle concentration could be modified.
The inventive construction leads to an aerosol generator, particularly for producing pure carbon aerosol, but also for producing aerosols of metals or inert gas or metal oxides when using corresponding metal electrodes, whose burn-up in the latter case is oxidized in an oxygen atmosphere as the propellant gas, which enables calibrations of measuring devices, filter tests and inhalation experiments to be carried out easily and very accurately.